1. Field of the invention
The present invention relates to pressure sensors for measuring body pressure at a selected site, and more particularly to an implantable pressure sensor which can be interrogated by X-ray of the body in which it is implanted to observe changes in pressure therein.
2. Description of the contemporary and/or prior art
There are several situations where the monitoring of pressure is necessary. In some of these situations, particularly in the biomedical arts, it is desirable to have a device which is entirely implantable within a body. For instance, it is frequently desirable to monitor ventricular pressure in hydrocephalics so that cerebrospinal fluid (CSF) can be drained from the ventricle if necessary or appropriate, or so that drug therapy can be initiated. Many prior art devices have been proposed which are transcutaneous, i.e., a portion thereof extends through the scalp of the patient to an appropriate readout device. The major drawback of these devices is the chance of infection at the site where the device breaches the scalp and the severe limitation on mobility of the patient.
Others in the art have sought to avoid this problem by using various fully implantable electronic devices which are interrogated by induction or which transmit coded information to an appropriate monitor. Aside from the requirement of having complex precision electronic equipment which must be implanted inside the head of a patient and the attendant cost, sophisticated monitoring apparatuses must also be employed. In addition, many of these apparatuses measure pressure across the dura rather than in the ventricle, a pressure, which in many medical circles, is not considered to be the same as ventricular pressure.
The treatment of hydrocephalus frequently involves implantation of a ventricular shunt and flushing valve arrangement for draining cerebrospinal fluid. None of the apparatuses presently known for monitoring and providing a readout of pressure are configured to be integratable with presently known shunt and flushing valve arrangements.
Of some of the known pressure monitoring devices, U.S. Pat. Nos. 3,977,391 and 4,124,023 issued to Fleischmann et al, and U.S. Pat. No. 4,006,735 issued to Hittman et al teach pressure sensing apparatuses wherein a tambour is exposed to pressure and a fluid in the tambour is forced thereout. In these apparatuses, this moving fluid is used to shift radioactive material relative to a shield in proportion to pressure changes so that the quantity of radioactive material can be statistically analyzed to determine relative pressure. While these configurations avoid the necessity of implanting electronics in the patient, a sophisticated monitoring apparatus is still needed to determine the amount of observed radioactivity.
Other pressure monitors which employ a sac or bladder filled with a fluid which is subjected to pressure include U.S. Pat. No. 3,911,902 issued to Delpy and U.S. Pat. No. 2,566,369 issued to Putman. These references teach the forcing of fluid through a calibrated tube so that pressure can be read by direct observation. Alternately, in Putnam, electrodes can be placed in the tube to determine position of the fluid. In Delpy, a liquid/gas interface shifts in a capillary tube thereby varying the capacitance between two wires disposed in the tube. By detecting changes in capacitance, a relative pressure can be indicated. Unfortunately, these apparatuses cannot be totally implanted and either the pressure readout scale or the wires of these apparatus must be transcutaneously positioned for readout. Therefore, the previously mentioned problems of immobility and infection exist.
The present invention overcomes the problems associated with the prior art by providing a totally implantable pressure sensor for measuring body pressure at a selected site within the body wherein a radiopaque material is mechanically shifted in proportion to changes in pressure. The subject or patient can then be X-rayed on widely available X-ray machines using known techniques to determine changes of pressure. This avoids the necessity of complex monitoring apparatuses or the use of transcutaneous configurations which not only subject the patient to a great risk of infection but also severely limit the mobility of the patient and therefore the possibility of long term pressure monitoring.
In several embodiments of the present invention this is accomplished through the use of a radiopaque fluid which shifts in position. Radiopaque fluids are known for use in variable pressure valves and are shown in U.S. Pat. Nos. 3,886,948 and 3,924,635 issued to Hakim. However, the radiopaque fluids in these apparatuses are used primarily for dampening and so that the position of the pressure sensing bladder of these devices can be determined by X-ray. Interrogating the relative position of the radiopaque fluid to determine changes in pressure are not shown or suggested and these devices merely use the shifting of the radiopaque fluid to trigger mechanical structure to perform the desired function.
In a further advance over the art, the present invention teaches the integration of a pressure monitoring device with a ventricular shunt. In a vaguely similar manner, U.S. Pat. No. 4,214,593 to Imbruce et al teaches an esophagal pressure device wherein a multiple lumen tube is employed, one of the lumens communicating with a balloon cup filled with a gas, the other lumen being used for typical nasogastric applications. As pressure acts on the balloon cuff, the gas is passed through the associated lumen so that changes in pressure can be monitored by an external monitor. This device is basically for transitory use and implantation is not shown or suggested.
Shifting of radiopaque material in a pressure monitor is shown in U.S. Pat. No. 4,172,449 issued to LeRoy et al. LeRoy teaches a body fluid pressure monitor wherein a radiopaque fluid disposed in a chamber distends the wall of the chamber, the curvature of the wall of the chamber showing relative changes in pressure. In another embodiment several radiopaque dots disposed on the outer surface of a balloon shift relative to each other as the balloon expands. Additionally, a Bourdon tube arrangement wherein a radiopaque marker shifts in response to pressure is also shown. Unfortunately, none of these configurations can supply precisely readable indications of pressure changes since the curvature of a membrane or the separation of radiopaque material in a non-linear manner is not easily calibratable when the angle at which the radiopaque material will be X-rayed cannot be exactly repeated. The present invention provides significant advantages over these configurations through the use of readily calibratable movement of radiopaque material which can read out giving direct quantitive indications of pressure changes.